1. Field of the Invention
The present invention relates to the field of silicon carbide composite materials and more specifically to a dense sintered silicon carbide composite body containing hexagonal phase boron nitride and carbon.
2. Description of the Related Art
Silicon carbide is an excellent material for mechanical and chemical applications. The physical and chemical properties of sintered silicon carbide include extreme hardness, high strength at room and elevated temperatures, low thermal expansion coefficient, good thermal shock resistance, good oxidation resistance and corrosion resistance. These characteristics render silicon carbide to be used extensively in demanding applications, such as components for gas turbines, chemical pump seals and bearings, anti-wearing nozzles, high temperature furnace fixtures, etc.
Despite the superior physical and chemical characteristics, the brittleness nature of sintered silicon carbide and lack of self lubricity, which often leads to catastrophic failure in critical applications, severely limits its application as an engineering material. To overcome such drawbacks, there have been a number of investigations into making silicon carbide-boron nitride composite materials that retain silicon carbide's exceptional resistance to oxidation and heat, high strength, extreme hardness and chemical inertness, and in the same time, provide better thermal shock resistance, improve toughness and self-lubricity.
With its superior adherence and thermo-chemical stability, hexagonal boron nitride powder retains its ability to lubricate under the most severe environments. It exhibits excellent resistance to oxidation, chemical attacks and high temperature stability up to 3000° C. Boron nitride has an oxidation threshold of approximately 850° C. and, even up to 1000° C., the rate of reaction is negligible.
Incorporation of boron nitride into silicon carbide matrix enhances the resulting composite's thermal shock resistance, provides self-lubricity in tribological applications at room and elevated temperatures, improves machinability and toughness. However, due to extreme inertness nature of boron nitride, incorporating meaningful amount of boron nitride into any engineering ceramics matrix, such as silicon carbide, silicon nitride or aluminum oxide, and still achieving high densities therefore maintaining composite material's physical integrity, has not been very successful. Hence, these materials invariably are prepared by hot pressing, as described in U.S. Pat. Nos. 3,954,483, 4,304,870 and 5,324,694. But hot pressing is an extremely expensive process and is only useful for producing simple shapes and has very limited commercial applications. Other methods of processing such material include in-situ reaction sintering, as disclosed in U.S. Pat. No. 6,764,974 and reaction bonding, as disclosed in U.S. Pat. No. 6,398,991. But these processes have severe limitations too. Reaction sintering is a slow and costly chemical process, and is not suitable for mass production. Reaction bonding results in a composite body that contains free silicon, thus limits its high temperature applications and has poor corrosion resistance.
To take full advantage of lubricating capability of boron nitride in silicon carbide body, the end products, such as seals or bearings, are usually machined to a very smooth surface, and in many cases, to mirror finish. It becomes very critical whether boron nitride inclusions in the sintered body can be retained on the surface during these material removal processes. Furthermore, the end products are often used in the most severe environments, subjecting to extreme heat, pressure, high speed erosion and chemical attack, therefore retaining boron nitride inclusions on the rubbing surface become even more difficult. Thus the bonding between powders that make up the granules, and the bonding of granules to the silicon carbide matrix become critical. U.S. Pat. No. 6,774,073 describes a process that uses temporary fugitive binder, such as polyvinyl alcohol, to prepare a dry lubricant(graphite) and fugitive binder mixture that is essentially spherical in shape, and then mix with silicon carbide. But such approach has severe consequences, during sintering, the fugitive binders evaporate, leaving no bonding force between the fine dry lubricant powders that make up the granules, and between the granules and the silicon carbide matrix. This is further illustrated in U.S. Pat. No. 5,395,807, wherein fugitive polymer spheres are used to create “controlled porosity” in sintered silicon carbide, and in U.S. Pat. No. 5,707,567, wherein polypropylene beads are used to create open porosities. Other important factors are the shape and surface morphology of boron nitride granules. During machining process to prepare the end use of the composite body, granules that have smooth surfaces are most likely to be pulled out of the matrix, while irregular, multi-faceted granules that have strong bonding within powders that make up granules are more likely to remain in the matrix and provide long term lubricating effects.
Thus, there exists a need for a dense silicon carbide body containing substantial amount of boron nitride that can be simply pressureless sintered. Furthermore, boron nitride incorporated should have good adhesion in the sintered body, thus during manufacturing process, or in actual rubbing applications under severe conditions, surface boron nitride can be retained.